Solid support-based chromatographic methods have been extensively applied in separation science for many years. [Harris, Quantitative Chemical Analysis, 2nd ed., W. H. Freeman and Co., New York (1987); Skoog, Principles of Instrumental Analysis, 3rd ed., Saunders College Publishing, New York (1985); Giddings, Unified Separation Science, John Wiley & Sons, New York, (1991)] Excellent chemical separations can be achieved due to the inherent variables of solid/liquid chromatography [Skoog, Principles of Instrumental Analysis, 3rd, ed., Saunders College Publishing, New York (1985)] that include the ability to vary both the support material and mobile phase. Several advantages over solvent extraction include the immobilization of the extractant and the absence (or decreased need in the case of extraction chromatography) of organic solvent diluents. As for solvent extraction, scale-up of solid support-based chromatographic methods is feasible with the major concern of the pressure drop across a large column balanced by the simplicity of the chromatographic apparatus versus liquid/liquid contactor apparatus.
Due to the rich history of liquid/liquid aqueous biphasic separations for biological separations, [Poly(Ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications; Harris, ed., Plenum Press, New York (1992); Aqueous Two-Phase Systems, Walter and Johansson, eds., in Methods in Enzymology, Academic Press, San Diego, Vol. 228 (1994); Albertsson, Partition of Cell Particles and Macromolecules, 3rd ed., John Wiley & Sons, New York (1986); Partitioning in Aqueous Two-Phase Systems. Theory, Methods, Uses and Applications to Biotechnology, Walter, Brooks and Fisher, eds., Academic Press, Orlando (1991)] work on solid-supported biphasic separations has focused on biological species. No work has been reported in the area of aqueous biphasic partitioning of metal ions using solid support separation particles.
The chelation properties of solid-supported short chain polyethers have been investigated, but the mobile phases were largely aqueous acids and the systems lacked genuine aqueous biphasic behavior. The chelation properties of the high molecular weight poly(ethylene glycols) (PEGs) is generally perceived to be quite low. Consequently, high molecular weight PEG resins have not been investigated for ion separations.
The major variables influencing liquid/liquid aqueous biphasic separations, type and concentration of polymers and salt and polymer molecular weight, are important variables to consider in the design of aqueous biphasic chromatographic materials. The current focus is on metal ion separations from solutions of high ionic strength because most metal-containing waste streams have relatively high concentrations of matrix ions.
Two major drawbacks to aqueous biphasic separations operating in the liquid/liquid mode are loss of the phase-forming components, PEG or salt, due to their high solubilities in water and the difficulty in stripping partitioned solutes. Because high concentrations of the phase-forming components are required to sustain a two-phase system, any loss of PEG or salt is of concern. In addition, different concentrations of phase-forming components have been shown to affect metal ion distribution ratios in liquid/liquid systems. [Rogers et al., Solvent Extr. Ion Exch., (in press 1995); Rogers et al., Aqueous Biphasic Separations: Biomolecules to Metal Ions, (in press 1995)]
More importantly, once the solute of interest has been partitioned to the upper PEG-rich phase of an aqueous liquid/liquid biphase, its isolation from this matrix has proven to be difficult. The back extraction conditions vary from chemical destruction of the extractant and partitioned complex to chemical reduction of the partitioned species [pertechnetate reduction by tin(II) chloride]. [Rogers et al., Solvent Extr. Ion Exch., (in press 1995); Rogers et al., Aqueous Biphasic Separations: Biomolecules to Metal Ions, (in press 1995)]
Polyethylene glycols have been bound to a variety of different materials, with the choice of support based primarily on the desired application. Solid-supported short chain PEGs have been grafted to Styrene-based resins for use as phase transfer catalysts in organic synthesis, [Regen et al., J. Am. Chem. Soc., 101:116 (1979); Yanagida et al., J. Org. Chem., 44:1099 (1979); Fukunishi et al., J. Org. Chem., 46:1218 (1981); Heffernan et al., J. Chem. Soc., Perkin Trans.2:514 (1981); Kimura et al., Synth. Commun., 13:443 (1983); Kimura et al., J. Org. Chem., 48:195 (1983)] and to urethane foams to act as potential metal ion chelators. [Jones et al., Anal. Chim. Acta, 182:61 (1986); Fong et al., Talanta, 39:825 (1992)] Polyethers have also been bound to various surfaces to decrease protein adhesion in biomedical applications [Nagaoka et al., Antithrombogenic Biomedical Material, Toray Industries, Inc. (1983); Toyobo Co., Antithrombogenic Membranes, Toyobo Co. (1983)] and medium weight PEGs have been fused to silica capillaries for a variety of separations. [Nashabeh et al., J. Chromatogr., 559:367 (1991); Herren et al., J. Coll. Interf. Sci., 115:46 (1987)] High molecular weight PEGs have been bound to silica [Matsumoto et al., J. Chromatogr., 187:351 (1980)] and Sepharose [Matsumoto et al., J. Chromatogr., 187:351 (1980); Matsumoto et al., J. Chromatogr., 268:375 (1981); Matsumoto et al., J. Chromatogr., 285:69 (1984)] primarily for polymer/polymer separations of biomolecules. Two recent reviews of PEG chemistry also point to the utility of solid-supported PEGs for bioanalytical separations. [Poly(Ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications, Harris ed., Plenum Press, New York (1992); Aqueous Two-Phase Systems, Walter et al., eds., in Methods in Enzymology Academic Press, San Diego, 228 (1994)] Each of the previously mentioned materials served as chelators or hydrophilic coatings to promote or inhibit different types of adsorption processes.
The importance of .sup.99m Tc in nuclear medicine and the problems associated with disposal of .sup.99 Tc in nuclear waste require new and better separations technologies for this element. In radiopharmacy, the short-lived .sup.99m Tc (t.sub.1/2 =6 hours) that decays to .sup.99 Tc (t.sub.1/2 =2.12.times.10.sup.5 years), is used in the vast majority of all medical procedures utilizing radioisotopes. [Boyd, Radiochim. Acta, 30:123 (1982); Steigman, The Chemistry of Technetium in Medicine, National Academy Press, Washington, D.C. (1992)]
One of the more common ways to access .sup.99m Tc is by eluting the pertechnetate ion (TcO.sub.4.sup.-1) from an alumina column containing .sup.99 MoO.sub.4.sup.-2 ion (t.sub.1/2 =66.7 hours), itself obtained by neutron activation irradiation of .sup.98 Mo or as a .sup.235 U fission product. So-called "instant technetium" involves the solvent extraction of .sup.99m TcO.sub.4.sup.-1 from an alkaline solution of Na.sub.2.sup.99 MoO.sub.4 using methyl ethyl ketone. Both methods suffer disadvantages including the presence of organic impurities and low radiochemical yield. [Boyd, Radiochim. Acta, 30:123 (1982); Steigman, The Chemistry of Technetium in Medicine, National Academy Press, Washington, D.C. (1992); Lamson et al., J. Nucl. Med., 16:639 (1975); Nair et al., Radiochim. Acta, 57:29 (1992)]
Relatively high levels of .sup.99 TcO.sub.4.sup.-1 are present in the highly alkaline waste storage tanks at Westinghouse Hanford [Fong et al., Talanta, 39:825 (1992)] and Savannah River [Walker et al., Mat. Res. Soc. Symp. Proc., 44:805 (1985)], among others. Technetium-99 is a fission product in nuclear fuel burn-up. Its long half life and its environmental mobility (as TcO.sub.4.sup.-1) present long term storage problems [Mobius, et al , "Gmelin Handbook of Inorganic Chemistry, Tc, Technetium: Metal Allgys, Compounds, Chemistry in Solution", 8th ed Supplemental vol 2 p 243 Kugler & Kellar, eds., Springer-Verlag, Berlin (1983); Jones, "Comprehensive Coordination Chemistry", Vol 6 p 881, Wilkinson et al., eds., Pergamon Press, Oxford (1987)]
Current extraction technologies for Tc run the gamut from solvent extraction to ion exchange in batch and chromatographic separations, and precipitation reactions. [Mobius, et al., "Gmelin Handbook of Inorganic Chemistry, Tc, Technetium: Metal Alloys, Compounds, Chemistry in Solution", 8th ed., Supplemental vol. 2, p. 243, Kugler & Kellar, eds., Springer-Verlag, Berlin (1983)] The synthetic organic reagents or resins used are often subject to radiation damage (in high level nuclear waste applications) and large cations (e.g., UO.sub.2.sup.+2, Zr.sup.+4) can be coextracted. [Mobius, et al., "Gmelin Handbook of Inorganic Chemistry, Tc, Technetium: Metal Alloys, Compounds, Chemistry in Solution", 8th ed., Supplemental vol. 2, p. 243, Kugler & Kellar, eds., Springer-Verlag, Berlin (1983); Jassim et al., Solvent Extr. Ion Exch., 2:405 (1984); Kolarik et al., Solvent Extr. Ion Exch., 7:625 (1989)] New separations techniques and tailored waste forms are needed for selective removal and immobilization of .sup.99 Tc.
The pertechnetate ion partitions to the polymer-rich phase in liquid/liquid PEG-based aqueous biphasic systems from a variety of salt solutions including OH.sup.-1, CO.sub.3.sup.-2, SO.sub.4.sup.-2 and PO.sub.4.sup.-3. Increasing the incompatibility between the two phases forces more of the TcO.sub.4.sup.-1 into the PEG-rich phase. This can be accomplished either by increasing the salt concentration or increasing the PEG-2000 concentration from about 20 weight percent to about 70 weight percent of the aqueous solution.
It would therefore be beneficial if the selective binding of TcO.sub.4.sup.-1 ions to PEG resins found in aqueous hipbasic separations could be adapted to a solid support-based separation and recovery process, while at the same time, overcoming the problems inherent in recovering the TcO.sub.4.sup.-1 ions from an aqueous bipbasic separation system using a solid phase that is not adversely affected by radiation present. The discussion that follows provides one solution to the TcO.sub.4.sup.-1 recovery problem.